Pneumatic impact mechanisms typically employ a reciprocating piston which is accelerated in one direction by means of alternately applied air pressure. The piston, upon striking its intended anvil (usually a chisel or the like), rebounds in the opposite direction and the process is repeated. Typically in pneumatic tools a bridge or stop is incorporated in the tool barrel to ensure that the piston will not be propelled out of the tool accidentally in the event the chisel is removed. In addition, when the tool is operated in the play-off mode with a long travel retainer (for example, when a typical pneumatic impact tool, such as a chipper) is removed from the work, the chisel moves forward allowing the piston to travel past the design strike point and to hit the retaining bridge.
Repeated hitting of the bridge creates high vibration levels and leads to eventual tool failure. To prevent the piston from striking the bridge, it is common to utilize an air cushion which is formed when the forward motion of the piston takes it past the strike point. The piston is stopped gradually in this manner without hitting the bridge.
Air cushions have been used on light duty pneumatic tools, such as scalers, and in heavy duty tools, such as rock drills. The formation of a successful air cushion requires tight fits between the piston diameters and the barrel bores and tight concentricity tolerances on barrel bores. These tight tolerances are difficult to hold, expensive to produce, and increase the rejection rate.
The present invention allows the use of conventional barrels with little modification and provides the required sealing for an effective air cushion.
It is an object of this invention to provide an air cushion seal which is self-aligning, inexpensive to manufacture, and effective without the need for tight concentricity sealing tolerances.
These and other objects are obtained in an air cushion seal for pneumatic impact tools having a reciprocating piston with a cushion end disposed in a barrel cylinder having a bridge for retaining the piston at one end comprising: a seal bushing disposed in close fitting concentric relationship with the cushion end of the piston when the piston approaches the bridge; the seal bushing being further disposed in concentric relationship with the barrel cylinder at the bridge and having minimum but appreciable radial clearance so as to allow the seal bushing to align itself with the cushion end of the piston while providing a substantially restricted diametral flow path of substantially increased length passed the bridge.